In severely abrasive conditions, protecting working surfaces is essential in many applications, such as with earth-moving equipment, to facilitate proper functioning. For example, when shoveling oil sand, the shovel teeth need an abrasion-resistant overlay for protection against excessive wear. Such overlays are typically a composite of hard particles, such as tungsten carbide, in a nickel alloy matrix, which are typically applied by welding.
Plasma transferred arc welding (PTA) is often chosen to deposit abrasion-resistant overlays because it is capable of producing high quality overlays with high efficiency. In addition, this welding technique can use a variety of materials, often in powder form, as consumables.
A standard PTA welding operation is shown in FIG. 1. A plasma arc is established between the electrode 12 and the workpiece 13 by ionizing the main gas, which is typically argon. Powder 14 is fed with an inert carrying gas through the nozzle 15 to the plasma arc 11, at which point it is melted and deposited onto the workpiece 13. An inert shielding gas 16 is also fed through a porous “diffuser” to the welding zone to minimize oxidation.
The most commonly used material blends or composite powders comprise (a) nickel alloy powder comprising silicon and boron, and (b) a tungsten carbide powder. Such powders are “self-fluxing,” and have the following two advantages:    (1) they have a relatively low melting point such that heat input is reduced, thereby avoiding excessive dilution with the base metal, which deteriorates the overlay's properties; and    (2) they yield overlay alloys with good abrasion resistance because hard phases of borides and silicides are formed therein.
However, there remain several deficiencies with traditional PTA welding when the described common material blends or composite powders are used to form weld overlays. First, the powder particles tend to adhere to or collect on the spray nozzle and the diffuser within a short time of beginning operation. Consequently, welding can only be done intermittently, with frequent stops required to remove the material from the equipment. The welding operation's efficiency is therefore reduced, and the final overlay has an increased propensity to form defects that originate when the weld operation is re-started.
Second, because tungsten carbide has a higher density than that of the nickel alloy matrix, it tends to settle to the bottom of the weld pool before the overlay solidifies. This results in a heterogeneous distribution of hard tungsten carbide particles, where the top portion of the overlay has fewer particles to promote adequate wear resistance in the overlay.
Finally, there is often an undesirable reaction between the tungsten carbide and nickel alloy in material blends comprising both powders. This reaction is further evident when chromium is present and/or when the tungsten carbide is a eutectic carbide, namely WC+W2C, which is less stable than WC. Specifically, chromium reacts with W2C to form a much softer chromium-rich complex carbide, e.g., M23C6, and W that is easily oxidized. Such a reaction is detrimental to the alloy's wear resistance.